1. Field of the Invention
The present invention relates to a process for producing a magnetic recording medium in which magnetic granules are isolated magnetically from each other effectively.
2. Related Background Art
Conventional magnetic recording media for hard disks are of a longitudinal magnetic recording type in which magnetization is recorded in the direction parallel to the disk face. In the longitudinal recording, for higher recording density, the magnetic recording layer is made thinner to produce the magnetic field outside above the recording medium for attenuating the diamagnetic field in the magnetic domains and for detecting the magnetization state. This requires extremely small size of the individual magnetic fine granule, tending to bring about a superparamagnetism effect. Thereby, the energy for stabilizing the magnetization direction becomes smaller than the thermal energy to cause change of the recorded magnetization with lapse of time, resulting in extinction of the magnetic record. Therefore, in recent years, the perpendicular magnetic recording system in which the recording layer can be made thicker is investigated actively to replace the conventional longitudinal magnetic recording system.
The media for the perpendicular magnetic recording have generally a recording layer containing a Co—Cr alloy formed by sputtering. In the layer formation, core portions containing more Co and a shell portion containing more Cr and surrounding the core portions grow separately.
However, in the Co—Cr system, the core portions of a high Co content can not be readily made finer for higher recording density. To offset such disadvantage, L10-ordered alloys of CoPt and FePt as shown in FIG. 10 are attracting attention in recent years. In FIG. 10, the reference numerals denote the followings: 1, a (001) plane; 2, a normal line for the (001) plane; 3, the direction of c-axis; 4, an element-a; 5, an element-b. For ordering the alloy, the heat treatment of the alloy should be conducted in a granular film state. In the heat treatment for ordering the alloy, coalescence of the magnetic granules occur concurrently. Before the heat treatment, magnetic granules of not larger than 10 nm diameter are isolated by a nonmagnetic matrix portion. In the heat treatment, however, the magnetic granules come to coalesce to form magnetic granules of diameters of several tens of nm, not achieving the marked improvement of performance by finer magnetic granules.
A technique is disclosed for ordering the alloy at a lower temperature (Japanese Patent Application Laid-Open No. 2004-311607: Patent Document 1), in which lower-temperature treatment is realized by addition of Cu to the FePt.
In the technique of the above Patent Document 1, since FePt and Cu are filled into the fine pores by metallizing, contamination by impurity tends to occur not to improve the magnetic properties as expected.
The present invention intends to provide a magnetic recording medium which is contaminated less and has improved magnetic properties.